Method and apparatus for applying electrode mixture paste

ABSTRACT

An electrode mixture paste is applied on both sides of a strip of core material made of porous metal sheet that is running along its lengthwise direction. The paste-coated core material is pressed with press rollers alternately from opposite sides once or more, after which the paste-coated core material is passed through the gap between a pair of scraper tools to adjust the coating thickness of the paste, while the position of the core material is restricted with comb-shaped projections, so that, even though the projections are distanced from each other sufficiently to allow weld points of the core material to pass through, the coating thickness is precisely regulated.

TECHNICAL FIELD

The present invention relates to a method and an apparatus for applyingelectrode mixture paste, and in particular, to a method and an apparatusfor applying electrode mixture paste with which a core material made ofporous metal sheet can be precisely coated with paste on both sides to arequired thickness and with which production failures can be prevented.

BACKGROUND ART

Alkaline rechargeable batteries and lithium ion rechargeable batteriesare widely used as the power source of portable devices, electric tools,or electric vehicles. Nickel metal hydride rechargeable batteries, amongothers, have a relatively high energy density and excellent durability,and have found increasing applications particularly in electric vehiclepower source systems.

The main compositions of nickel metal hydride rechargeable battery are,generally, a positive electrode formed of a three-dimensional porousmetal body filled with nickel hydroxide and other components, and anegative electrode formed of a core material which is a porous metalsheet such as punched metal and electrode mixture paste composed ofhydrogen-absorption alloy and other components applied on the corematerial. Of these two, the negative electrode is attracting attentionbecause of its potential for continuous production and possibilities ofhighly efficient manufacture. More specifically, a known technique foradjusting the coating thickness of electrode mixture paste is, afterimmersing the core material in electrode mixture paste to apply thepaste on both sides of the core material, to pass the paste-coated corematerial through a gap between a pair of scraper tools, the surface ofthe core material making contact with several projections provided tothe scraper tools to restrict the position of the core material so thatit passes through the center between the tools. (see, for example,Patent Document 1).

-   [Patent Document 1] Japanese Patent Laid-Open Publication No.    9-134722

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

When using the above batteries as the power source of electric vehicles,one car requires 100 to 200 cells connected in series to achieve highvoltage output. Manufacture of a large amount of such electrodes isdesigned to include a fewer number of times for adjusting the coatingthickness of electrode mixture paste, for example by welding togetherseveral hundred meter long core materials during production to enablecontinuous processing of the core materials over a length of severalkilometers.

However, with the application method disclosed in Patent Document 1, thesize of the gap between the pair of scraper tools determines the coatingthickness to which it is adjusted, and the position of the core materialis restricted by the projections provided to the scraper tools makingcontact with the surface of the core material. The problem is that whena weld point that has a doubled thickness passes between the scrapertools, the weld point gets stuck between the projections of the scrapertools, because of which the application process has to be stopped.

If, to solve this problem, the distance between the projections of thepair of scraper tools were increased to equal to or more than twicelarger than the thickness of the core material, the running corematerial would be displaced, making it impossible to adjust the coat onboth sides to a uniform thickness. In an electric vehicle battery, asmentioned above, 100 to 200 cells are connected in series, and loweredcoating thickness precision would cause capacity variation among cells,which may lead to failures such as generation of gas or heat inlow-capacity cells.

In light of the problems in the conventional techniques mentioned above,an object of the present invention is to provide a method and anapparatus for applying electrode mixture paste with which coatingthickness is precisely adjusted and, even in a continuous productionprocess in which core materials are connected together by welding,production failures due to weld points being stuck are prevented.

Means for Solving the Problems

To achieve the above object, a method for applying an electrode mixturepaste according to the present invention includes: an application stepof applying an electrode mixture paste on both sides of a strip of corematerial made of porous metal sheet as the core material runs along alengthwise direction thereof; a uniformizing step of pressing the corematerial coated with the electrode mixture paste alternately fromopposite sides once or more; and a coating thickness adjusting step ofadjusting a coating thickness of the electrode mixture paste by passingthe core material coated with the electrode mixture paste through a gapbetween a pair of scraper tools, a position of the core material beingrestricted using comb-shaped projections provided to at least one of thescraper tools.

With this method, as the core material coated with the electrode mixturepaste is pressed alternately from opposite sides, the applied paste isrepeatedly pushed out through the pores in the core material, wherebyair bubbles in the applied paste are removed and paste properties aremade uniform. This enables precise position restriction of the corematerial with the comb-shaped projections equally working on both sides,even though the comb-shaped projections of the following scraper toolsare distanced from each other by a length equal to or more than twicelarger than the thickness of the core material so that the weld pointsof the core material passes so smoothly through the gap between thescraper tools. As a result, the coating thickness is precisely adjustedand, even in a continuous production process in which core materials areconnected together by welding, production failures due to weld pointsbeing stuck are prevented.

The comb-shaped projection that is provided to one scraper tool is madecontact with one surface of the core material to restrict the positionof the core material so as to achieve precise position control of thecore material. The increased running stability ensures preciseregulation of the coating thickness.

A press member that is nearest to the pair of scraper tools may beoffset from a centerline of the gap between the scraper tools along arunning direction of the core material towards a side of one scrapertool that is provided with the comb-shaped projections, so as to ensurethat the core material runs in contact with the comb-shaped projectionsof one scraper tool, whereby the coating thickness is adjusted stablyand precisely.

The present invention also provides an apparatus for applying anelectrode mixture paste including: an uncoiler unit for unwinding a corematerial from a coil thereof; a coating unit where the core material isimmersed in an electrode mixture paste to coat the core material withthe paste on both sides thereof; a coating thickness adjusting unit foradjusting a coating thickness of the electrode mixture paste; a dryingunit for drying the electrode mixture paste; and a coiler unit fortaking up the core material into a coil on which the electrode mixturepaste has been applied and dried, wherein the coating thicknessadjusting unit includes at least one pair of press members for pressingthe core material coated with the electrode mixture paste alternatelyfrom opposite sides, and a pair of scraper tools, at least one of whichincludes a comb-shaped projection for restricting a position of the corematerial, the core material coated with the electrode mixture pastebeing passed through a gap between the scraper tools.

With this structure, by setting coils of core material in the uncoilerunit and starting the application apparatus, the coiler unit producescoils of finished core material on which the electrode mixture paste hasbeen applied and dried, as the electrode mixture paste is applied onboth sides of the core material to a precisely controlled thickness, andeven in a continuous production process in which core materials areconnected together by welding, production failures due to weld pointsbeing stuck are prevented, by the above application method.

The comb-shaped projection which is made contact with one surface of thecore material to restrict the position of the core material may beprovided to one scraper tool, and the press member that is nearest tothe scraper tools may be offset from a centerline of the gap between thescraper tools along a running direction of the core material towards theside of one scraper tool that is provided with the comb-shapedprojection, so as to ensure that the core material runs in is contactwith the comb-shaped projections of one scraper tool, whereby thecoating thickness is adjusted stably and precisely.

A welding unit for connecting core materials together may be set betweenthe uncoiler unit and the coating unit so that the core materials arewelded together during the operation of the application apparatus forcontinuous production, whereby the number of times for adjusting thecoating thickness is reduced and the production efficiency improved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic perspective view illustrating the structure of afirst embodiment of an apparatus for applying electrode mixture paste inaccordance with the present invention;

FIG. 2 is a side view illustrating the structure of the coatingthickness adjusting unit in the same embodiment;

FIG. 3 is a partial perspective view of one of the scraper tools in thesame embodiment;

FIG. 4 is a side view illustrating the structure of the coatingthickness adjusting unit in accordance with a second embodiment of thepresent invention;

FIG. 5 is a side view illustrating the structure of the coatingthickness adjusting unit in accordance with a third embodiment of thepresent invention; and

FIG. 6 is a side view illustrating the structure of one example ofcoating thickness adjusting unit given for comparison.

BEST MODE FOR CARRYING OUT THE INVENTION

One embodiment of the method and apparatus for applying electrodemixture paste of the present invention will be hereinafter describedwith reference to FIG. 1 to FIG. 5.

First Embodiment

A description will now be given of a first embodiment of the presentinvention with reference to FIG. 1 to FIG. 3.

FIG. 1 is a perspective view illustrating the schematic overallstructure of an application apparatus 100 according to the presentembodiment. The application apparatus 100 includes an uncoiler unit 3which holds a coil of core material 2 prepared by winding a strip of acore material 1 made of porous metal sheet and unwinds the strip of corematerial 1.

The core material 1 is drawn out horizontally from the uncoiler unit 3,turned about 90° around a guide roller 4 to be directed upwards and torun into a coating unit 7. The coating unit 7 includes a paste bath 6filled with an electrode mixture paste 5, with the core material 1 beingarranged to run through the paste bath 6, so that the core material 1 iscoated with the electrode mixture paste 5 on both sides in this coatingunit 7.

The core material 1 coated with the electrode mixture paste 5 runsapproximately upwards and passes through a coating thickness adjustingunit 8 where the coating thickness of the paste 5 is adjusted, and comesout as a paste-coated sheet 10. The paste-coated sheet 10 turns about90° around a guide roller 9 to be directed to the horizontal direction,runs into a drying unit 11 where the electrode mixture paste 5 is dried,and comes out as an electrode sheet 12, which is taken up in a coilerunit 13 to produce a coil of electrode 14.

Welding means (not shown) is arranged between the uncoiler unit 3 andthe coating unit 7 so that, when the coil 2 of core material 1 runs outand another coil 2 of core material 1 is placed in the uncoiler unit 3,the ends of the core materials 1 are welded together to enablecontinuous production. Seam welding, which uses copper electrodes, ispreferably used as the welding means.

As shown in FIG. 2 in detail, the coating thickness adjusting unit 8 iscomposed of one or a plurality of pairs of press rollers 15 a and 15 bthat alternately press one surface each of the core material 1 coatedwith the electrode mixture paste 5 from opposite sides, and a pair ofscraper tools 16 a and 16 b for adjusting the coating thickness of thepaste 5 as the paste-coated core material 1 passes through the gapbetween them. Of the pair of scraper tools 16 a and 16 b, one scrapertool 16 a is formed with a plurality of thin-plate projections 17 a in acomb arrangement as shown in FIG. 3 to restrict the position of the corematerial 1 by making contact with the surface of the running corematerial 1. The other scraper tool 16 b may also have comb-shapedprojections, in which case the tips of the projections should bedistanced from each other by a length that is equal to or more thantwice larger than the thickness of the core material 1.

In the present embodiment, the press roller 15 b nearest to the scrapertools 16 a and 16 b is offset from the centerline 18 of the gap betweenthe scraper tools 16 a and 16 b along the running direction of the corematerial towards the side of the scraper tool 16 a that has theprojections 17 a. The scraper tool 16 a that has the projections 17 a isarranged movable, while the other scraper tool 16 b is fixedly set, sothat the distance between the scraper tools 16 a and 16 b is adjustableby changing the position of the scraper tool 16 a to regulate thecoating thickness. Of course, both of the scraper tools 16 a and 16 bmay be arranged movable for the position adjustment.

How this coating thickness adjusting unit 8 works will be described nextwith reference to FIG. 2. The core material 1 coated with the electrodemixture paste 5 is first pressed from one side by the press roller 15 ato push the paste 5 through the pores in the core material 1 to theopposite side of the press roller 15 a. The core material 1 is thenpressed from the opposite side by the press roller 15 b to push thepaste 5 through the pores in the core material 1 to the opposite side ofthe press roller 15 b. This process of pushing the paste 5 through thepores in the core material 1 alternately from opposite sides isperformed at least once or repeated twice or more so as to remove airbubbles in the applied paste and to make the paste properties uniform.

Successively, the core material 1 coated with the electrode mixturepaste 5 passes through the gap between the pair of scraper tools 16 aand 16 b. Since the press roller 15 b nearest to the scraper tools 16 aand 16 b is offset from the centerline 18 of the gap towards the scrapertool 16 a that has the projections 17 a, it is ensured that the corematerial 1 runs in contact with the projections 17 a, and it runs stablywith its position being restricted.

The core material 1 coated with the electrode mixture paste 5 in thisstate enters the gap between the pair of scraper tools 16 a and 16 b,whereby the paste 5 that has been pushed out by the press roller 15 b ispressed by the other scraper tool 16 b and pushed out through the poresin the position-restricted core material 1 to the side of the scrapertool 16 a. After that, finally, the pair of scraper tools 16 a and 16 bscrape off redundant electrode mixture paste 5. Thus, the core material1 which is precisely positioned is coated with the electrode mixturepaste 5 on both sides to a required thickness with uniform pasteproperties, whereby a paste-coated sheet 10 with the coating ofprecisely controlled thickness is produced.

The projections 17 a provided to one scraper tool 16 a are distancedfrom the end of the other scraper tool 16 b by a length that is morethan twice larger than the thickness of the core material 1 so that,even if the weld point where the ends of the core materials are weldedtogether has a thickness twice larger than the thickness of the corematerial 1, the weld point will not be stuck in between the scrapertools 16 a and 16 b but pass through smoothly, and will not causeproduction failures.

Second Embodiment

Next, a second embodiment of the present invention will be describedwith reference to FIG. 4. The same elements as those of the firstembodiment are given the same reference numerals and will not bedescribed again, and the difference only will be described.

In the first embodiment described above, the press roller 15 b nearestto the scraper tools 16 a and 16 b is offset from the centerline 18 ofthe gap between the scraper tools 16 a and 16 b to the side of onescraper tool 16 a. In the present embodiment, the press roller 15 b ispositioned substantially on the centerline 18.

With the present embodiment, too, the core material 1 and the electrodemixture paste 5 that has been pushed out by the press roller 15 b arepressed against the scraper tool 16 a by the other scraper tool 16 b,whereby the core material 1 makes contact with the projections 17 a andits position is restricted. Thus, the core material 1 which is preciselypositioned is coated with the electrode mixture paste 5 on both sides toa required thickness with uniform paste properties, whereby apaste-coated sheet 10 with the coating of precisely controlled thicknessis produced. Also, even if the weld point where the ends of the corematerials 1 are welded together has a thickness twice larger than thethickness of the core material 1, the weld point will not be stuck inbetween the scraper tools 16 a and 16 b but pass through smoothly, andwill not cause production failures.

Stability is better in the first embodiment because the core material 1is more reliably brought into contact with the projections 17 a.

Third Embodiment

Next, a third embodiment of the present invention will be described withreference to FIG. 5. The same elements as those of the first and secondembodiments are given the same reference numerals and will not bedescribed again, and the difference only will be described.

In the present embodiment, a plurality of press rollers 15 c and 15 dare provided for adjusting coating thickness between the press roller 15b of the second embodiment described above and the scraper tools 16 aand 16 b, so that the electrode mixture paste 5 is applied on both sidesof the core material 1 with a substantially uniform thickness. The pairof scraper tools 16 a and 16 b each have a plurality of projections 17 aand 17 b in a comb arrangement for restricting the position of the corematerial 1. The projections 17 a and 17 b are adjusted so that theirtips are distanced from each other by a length that is equal to or morethan twice larger than the thickness of the core material 1.

With the present embodiment, the process of pushing out the electrodemixture paste 5 through the pores in the core material 1 alternately bythe press rollers 15 a and 15 b is performed at least once or repeatedtwice or more so as to remove air bubbles in the applied paste and tomake the paste properties uniform, after which the press rollers 15 cand 15 d make the coating thickness of the paste 5 substantially uniformon both sides of the core material 1. The core material 1 in this stateenters the gap between the scraper tools 16 a and 16 b, whereby, whilethe projections 17 a and 17 b restrict the position of the core material1 indirectly through the uniform paste interposed in the small gapbetween the tips of the projections 17 a and 17 b and the core material1, the pair of scraper tools 16 a and 16 b scrape off redundantelectrode mixture paste 5 from both sides of the core material 1. Thus,the core material 1 which is precisely positioned is coated with theelectrode mixture paste 5 on both sides to a required thickness withuniform paste properties, whereby a paste-coated sheet 10 with thecoating of precisely controlled thickness is produced.

The projections 17 a and 17 b provided to the scraper tools 16 a and 16b are distanced from each other by a length that is equal to or morethan twice larger than the thickness of the core material 1, so that,even if the weld point where the ends of the core materials 1 are weldedtogether has a thickness twice larger than the thickness of the corematerial 1, the weld point will not be stuck in between the projections17 a and 17 b but pass through smoothly, and will not cause productionfailures.

As is clear from the description of the embodiment above, the presentinvention can be applicable to any core material of porous metal sheetand is not limited to production of negative electrodes. For example,the invention is applicable to production of sintered substrates whichare a precursor of sintered nickel positive electrode of an alkalinerechargeable battery, or positive or negative electrodes of lithiumpolymer batteries that use metal lath, which is a porous metal sheet.

The following is a description of working examples in which the presentinvention is applied to the negative electrode (hydrogen-absorptionalloy electrode) of nickel metal hydride rechargeable battery, andcomparative examples.

Example 1

Hydrogen-absorption alloy expressed by the composition formulaMmNi_(3.55)Co_(0.75)Mn_(0.4)Al_(0.3) was pulverized in water by wet ballmill to achieve hydrogen-absorption alloy powder with a mean particlediameter of 30 μm. The hydrogen-absorption alloy powder was immersed inan alkaline aqueous solution for surface treatment, after which 10 kg ofaqueous carboxymethyl cellulose solution with a solid component ratio of1.5% and 0.4 kg of Ketjen black were added to 100 kg of thehydrogen-absorption alloy powder and kneaded, and then 1.75 kg ofstyrene-butadiene copolymer rubber particles in aqueous solution with asolid component ratio of 40% was added and stirred, to produce electrodemixture paste.

The electrode mixture paste was applied on both sides of a core material1 made of nickel-plated punched iron sheet with a thickness of 60 μm, awidth of 300 mm, a punched hole diameter of 1 mm, and a pore rate of40%, at a speed of 5 m/min (total length of the coil 2 of core material:200 m), such that the coating width will be 260 mm and the total coatingthickness will be 260 μm, while scraping off the paste 5 from 20 mm widemargins at both side edges. More specifically, the core material 1 andother units were arranged as shown in FIG. 2, the protruding length ofthe projections 17 a being 100 μm, and the distance between theprojections 17 a and the scraper tool 16 b being 160 μm. Paste wasapplied over a length of 1000 m, and the core materials were weldedtogether four times during the process. The core material 1 did not onceget stuck in between the scraper tools 16 a and 16 b during the pasteapplication.

Samples were collected from the paste-coated sheet 10 by punching aportion at every 1 m using a 40 mm diameter puncher. The averagedifferences in coating thickness between the front and back sides ofthese samples were determined for every 200 m or for each one of fivecoils of core material (200 data samples per one coil). According to theresults, the average thickness differences of the five coils were 1 μm,3 μm, 2 μm, 4 μm, and 2 μm, respectively, in order from the start ofpaste application.

Example 2

The electrode mixture paste 5 was applied similarly to Example 1, theonly difference from Example 1 being that the core material 1 and thepress roller 15 b were in contact with each other as shown in FIG. 4.Similarly to Example 1, the core material 1 did not once get stuck inbetween the scraper tools 16 a and 16 b during the paste application.

Samples were taken from the paste-coated sheet 10 similarly to Example1, and the average differences in coating thickness between the frontand back sides of these samples were determined for each coil of corematerial. According to the results, the average thickness differences ofthe five coils were 6 μm, 3 μm, 5 μm, 6 μm, and 4 μm, respectively, inorder from the start of paste application.

Example 3

The core material 1 and various units were arranged as shown in FIG. 5,with the protruding length of both projections 17 a and 17 b being 70 μmand the projections 17 a and 17 b being distanced from each other by 120μm. With other conditions being the same, the electrode mixture paste 5was applied similarly to Example 1. Similarly to Example 1, the corematerial 1 did not once get stuck in between the scraper tools 16 a and16 b during the paste application.

Samples were taken from the paste-coated sheet 10 similarly to Example1, and the average differences in coating thickness between the frontand back sides of these samples were determined for each coil of corematerial. According to the results, the average thickness differences ofthe five coils were 15 μm, 20 μm, 18 μm, 13 μm, and 16 μm, respectively,in order from the start of paste application.

Comparative Example 1

The press rollers 15 a to 15 d were removed from the arrangement ofExample 3, and the core material 1 was fed as shown in FIG. 6. Withother conditions being the same, the electrode mixture paste 5 wasapplied similarly to Example 1. Similarly to Example 1, the corematerial 1 did not once get stuck in between the scraper tools 16 a and16 b during the paste application.

Samples were taken from the paste-coated sheet 10 similarly to Example1, and the average differences in coating thickness between the frontand back sides of these samples were determined for each coil of corematerial. According to the results, the average thickness differences ofthe five coils were 36 μm, 28 μm, 41 μm, 32 μm, and 45 μm, respectively,in order from the start of paste application.

Comparative Example 2

The electrode mixture paste 5 was applied similarly to ComparativeExample 1, the only difference from Comparative Example 1 being that theprojections 17 a and 17 b of the scraper tools 16 a and 16 b weredistanced from each other by 65 μm. As the first weld point where thecore materials are welded together passed between the scraper tools 16 aand 16 b, the core material 1 got stuck and so the paste application wasstopped. Samples were taken from the paste-coated sheet 10 similarly toExample 1, and the average difference in coating thickness between thefront and back sides was determined for the first coil of core material.According to the results, the to average thickness difference was 2 μm.

As is clear from the above description, the application method of thepresent invention proved effective in reducing variation in coatingthickness between the front and back sides of the core material as inComparative Example 1, and in preventing failures resulting from weldpoints of the core material 1 as in Comparative Example 2. Examples 1and 2, in particular, showed that feeding the core material 1 in contactwith the projections 17 a of only one scraper tool 16 a improved thestability of the running core material 1, resulting in more precisecontrol of coating thickness. Moreover, it was ascertained that morepreferable results are achieved from the arrangement in which the pressroller 15 b nearest to the scraper tools 16 a and 16 b is offset fromthe centerline 18 of the gap between the scraper tools 16 a and 16 balong the running direction of the core material towards one scrapertool 16 a provided with the projections 17 a, as in Example 1, becausethe running core material 1 is reliably brought into contact with theprojections 17 a.

INDUSTRIAL APPLICABILITY

According to the method for applying electrode mixture paste of thepresent invention, the core material coated with the electrode mixturepaste on both sides is pressed alternately from opposite sides to makethe paste properties uniform, so that, even when theposition-restricting ends of the scraper tools are distanced from eachother by a length that is equal to or more than twice larger than thethickness of the core material, the position of the core material isprecisely restricted when the core material is passed through the gapbetween the scraper tools to adjust the coating thickness of the paste.Therefore, not only the coating thickness is precisely regulated, butalso, even in a continuous production process where core materials areconnected together by welding, production failures due to weld pointsbeing stuck are prevented. This enables precise mass production ofelectrodes of rechargeable batteries for electric vehicle applications,where a large number of cells need to be connected in series, and thusthe present invention is highly applicable to electrode productiontechniques of various batteries, and can be used in the production ofelectrodes of various batteries such as lithium ion batteries and nickelmetal hydride batteries.

1-3. (canceled)
 4. An apparatus for applying an electrode mixture paste,comprising: an uncoiler unit for unwinding a core material from a coilthereof; a coating unit where the core material is immersed in anelectrode mixture paste to coat the core material with the paste on bothsides thereof; a coating thickness adjusting unit for adjusting acoating thickness of the electrode mixture paste; a drying unit fordrying the electrode mixture paste; and a coiler unit for taking up thecore material into a coil on which the electrode mixture paste has beenapplied and dried, wherein the coating thickness adjusting unit includesat least one pair of press members for pressing the core material coatedwith the electrode mixture paste alternately from opposite sides, and apair of scraper tools, at least one of which includes a comb-shapedprojection for restricting a position of the core material, the corematerial coated with the electrode mixture paste being passed through agap between the scraper tools.
 5. The apparatus for applying anelectrode mixture paste according to claim 4, wherein the comb-shapedprojection which is made contact with one surface of the core materialto restrict the position of the core material is provided to one scrapertool and the press member that is nearest to the pair of scraper toolsis offset from a centerline of the gap between the scraper tools along arunning direction of the core material towards a side of one scrapertool that is provided with the comb-shaped projection.
 6. (canceled)